The Nigerian military says it is leading a manhunt for gunmen who seized two foreign oil workers in the latest attack on the country's troubled oil industry. Gilbert da Costa in Abuja reports that the kidnapping has heightened fears of possible violence after threats by a rebel group to resume attacks following the arrest of one its leaders.

A military spokesman in the oil-rich Niger Delta, Major Sagir Musa, says security agencies have been mobilized to track down the kidnappers and secure the release of two abducted oil workers.

"Efforts are being made by the JTF [Joint Military Task Force] and other relevant security agencies to ensure their early release," he said. "The management [of the Saipem oil company] has equally expressed disaffection about the whole situation and their readiness and commitment to ensuring that they are released immediately."

No group has claimed responsibility for Thursday's attack in the main oil city of Port Harcourt, in southern Nigeria.

Officials say 10 gunmen disguised as soldiers raided the construction yard of an oil service company Saipem. One Colombian worker was killed and a second Colombian worker and a Filipino were taken hostage.

It was the first high-profile attack on an oil company in the Niger Delta in more than two months.

A rebel group, the Movement for the Emancipation of the Niger Delta, or MEND, had on Sunday threatened to launch attacks on Africa's top oil producer following the arrest of one of its leaders in Angola.

Major Musa says the authorities are taking the threat seriously.

"This is not the first time such a group is making a threat," he said. "But we are not taking that as empty. We are monitoring the situation to avert all loopholes such kinds of people may use to perpetrate their evil acts."

Western oil companies have strengthened security in the wake of the threat.

Armed groups who claim to be seeking a larger share of the region's oil wealth have stepped up attacks on foreigners and oil facilities in the past two years.

More than 200 foreign workers have been abducted since early 2006. Nearly all have been released after the payment of ransom.